With regard to optical transmissions, there is demand for further improvement in the communication speed due to an increase in communication traffic. There are known technologies for improving the communication speed, such as Nyquist wavelength division multiplexing (WDM) for improving the frequency usage efficiency of an optical band.
The Nyquist-WDM is implemented by spectrum shaping using digital signal processing. For this spectrum shaping, filtering processing is performed on a wide-band frequency spectrum of a non-return-to-zero (NRZ) optical signal, or the like, so as to reduce the band of the frequency spectrum or shape it. With regard to the filtering processing, there is a well-known filter that makes a time response with a Sinc function form, such as a raised cosine filter and, due to convolution in NRZ optical signals, the band of the frequency spectrum is reduced, and it is shaped into a rectangle.
FIG. 14 is an explanatory diagram that illustrates a comparison between a conventional WDM (case C1) and the Nyquist-WDM (case C2). As illustrated in FIG. 14, in the case C2 of the Nyquist-WDM, optical signals of multiple subcarriers are subjected to wavelength multiplexing in a higher density compared to the case C1 of the conventional WDM. For example, the Nyquist-WDM employs the concept called super-channel in which the optical signals of subcarriers are multiplexed and they are regarded as a single optical signal.
Patent Document 1: Japanese Laid-open Patent Publication No. 2013-201495
Patent Document 2: Japanese Laid-open Patent Publication No. 8-293853
Furthermore, with regard to optical transmissions, if the wavelength interval between adjacent subcarriers is small, the effect of crosstalk between adjacent subcarriers is likely to occur, and the transmission performance is sometimes decreased (performance degradation). Moreover, if the wavelength interval between adjacent subcarriers is small, the performance degradation due to crosstalk sometimes occurs largely due to a wavelength displacement of an optical signal.
FIG. 15 is an explanatory diagram that illustrates a case where the wavelength interval is small in the Nyquist-WDM. As illustrated in FIG. 15, in a case C3 in which the wavelength interval of subcarriers is 50 GHz and a case C4 in which the wavelength interval of subcarriers is equal to or less than 50 GHz, the case C4 is more likely to be affected by a wavelength displacement of the light source. Therefore, there is a need to obtain the center wavelength of the frequency spectrum of an optical signal and adjust a wavelength displacement of the optical signal with high accuracy.
However, in the Nyquist-WDM, the frequency spectrum of an optical signal is shaped into a rectangle; therefore, it is difficult to determine the center wavelength by using the wavelength in which the intensity of the frequency spectrum has some local peaks. Thus, it is not easy to determine the center wavelength of a frequency spectrum with accuracy, and it is difficult to adjust a wavelength displacement of an optical signal with high accuracy.
According to one aspect, it is possible to adjust a wavelength displacement of an optical signal with high accuracy.